1. Field of the Invention
The present invention relates to a method for measuring a size of a multilayer structured container suitable for measuring a thickness of a layer or a space between layers of the multilayer structured container.
2. Description of the Related Art
In order to improve heat insulation effect or safety to leakage, explosion, or the like, a multiple (multilayer) structure vessel composed of different materials is often adopted for a storage container of liquid or gas. Furthermore, a space such as a vacuum layer, an air layer, or the like is often is provided between an outer layer (an outer wall) and an inner layer (an inner wall). For such a multilayer structured container, it has been desired that a size of a wall thickness of each layer or that of the space between layers can be easily measured in order to control quality of a manufacturing process or to monitor aging due to usage.
Furthermore, as one of hydrogen-tanks for a fuel cell of an automobile, a double-layer structured container covered with a carbon fiber outside of an aluminum container has been developed. A generation of the space between the layers due to the aging has been pointed out because the container has been exposed to repeated pressure change or temperature change, and therefore, it has been desired that the space therebetween can be easily measured.
When such a multilayer structured container is inspected, a non-explosive inspection is required, and conventionally, an X-ray inspection method is generally carried out.
As shown in FIG. 8, an inner structure of a multilayer structured container 2 is inspected by the conventional X-ray inspection method in such a manner that X-rays radiated conically from an X-ray tube 1 are irradiated on the multilayer structured container 2 as an object to be inspected, and that the X-rays transmitting the multilayer structured container 2 are detected by a film 3. According to the inspection method, for example, when a space 2C exists between an outer layer 2A and an inner layer 2B, an absorption degree becomes the smallest at a portion where the X-rays transmit the space 2C to be tangent to an outer periphery of the inner layer 2B; therefore, a space is confirmed to exist at a portion where intensity of transmitting X-rays is high based on a detected result by the film 3.
However, in the conventional X-ray inspection method, the X-rays diverged conically from the X-ray tube 1 is directly taken by the film 3; therefore, it is difficult to accurately measure even a size of the space 2C or wall thicknesses of the outer and inner layers 2A and 2B although existence of the space 2C between the outer and inner layers 2A and 2B is vaguely confirmed.